In recent years, there is a growing interest in the use of the semiconductor laser as the light source of image display systems. The semiconductor laser has various advantages over the mercury lamp which is commonly used as the light source for more conventional image display systems, such as a better color reproduction, the capability to turn on and off instantaneously, a longer service life, a higher efficiency (or a lower power consumption) and the amenability to compact design.
A known image display system using the semiconductor laser typically comprises three laser light source units of red, green and blue colors and a spatial light modulator consisting a liquid crystal display device, and the laser light of a different color produced from each laser light source unit is impinged upon the spatial light modulator in a sequence from one color to another for each frame in a time sharing manner (field sequential process). See JP 2010-091927A. The images of different colors that are projected onto a screen in a sequential manner are perceived by the viewer as a color image of a single frame owing to the afterimage effect. When a plurality of such color images or frames are displayed in a consecutive manner as a motion picture, the images are viewed by the viewer as a continuous color motion picture. According to this technology, as only one spatial light modulator is required, the display device can be constructed as a highly compact unit.
In such a time sharing display process, each frame is divided into a plurality of sub frames (lighting intervals) that are generated by the different laser light sources of red, green and blue colors, and the spatial light modulator controls the output of the laser light of each color in synchronism with the generation of sub frames. In particular, intermediate colors may be produced by combining the laser lights of different colors in the given frame. For instance, when yellow color is to be displayed, the red and green laser lights in two different sub frames are combined or mixed in the given frame.
The liquid crystal display device used for the spatial light modulator has a time delay in response, and the transmissivity thereof increases only gradually after applying a control voltage thereof. Therefore, when the laser lights of two different colors are emitted one after another to produce an intermediate color, the laser light of one of the colors that is emitted first gets diminished as compared to the laser light of the other color that is emitted later, and this causes the produced color to deviate from the intended intermediate color. For instance, when the laser light of yellow color is produced by first emitting red laser light and then emitting green laser light, as the red laser light is diminished by the delay in the response of the spatial light modulator, the produced laser light results in greenish yellow color because of the relative dominance of the green color. As red and green colors have relatively high luminosity factors, a color error in the intermediate color produced by the combination of red and green colors are particularly noticeable to human eyesight.
Furthermore, the green, red and blue colors of the laser lights generated by the green, red and blue laser light source units 22 to 24, respectively, may not be adequately close to the corresponding standard colors. In particular, according to the current technology, the green laser light generated by a green laser light source unit may deviate significantly from the standard green color. This also contributes to the undesired shifting of the color phases of intermediate colors.